This invention relates to the use of certain ester urethanes for retanning chrome-tanned leather. "Retanning" is understood to be the after-treatment of pretanned (generally chrome-tanned) leather in order to optimize color, levelness, softness, fullness, and behavior to water (hydrophobicity) and to fix tanning agents.
In addition to the group of so-called syntans, the most important retanning agents at present are carboxy-functional polymers in salt form; European Patent Applications 118,023 and 372,746 and German Offenlegungsschrift 3,931,039. However, polyurethanes optionally containing polyethylene oxide groups, ionic groups, or methylol groups have also been used for retanning; see German Offenlegungsschrift 2,504,081.
Hitherto known retanning agents do not satisfy all the expected requirements either because their hydophobicizing effect is inadequate or because they adversely affect feel, dyeability, grain pipeyness, or the subsequent finishing of the leather. The large number of desirable properties necessitates a compromise. Desirable retanning agents are those which optionally hydrophobicize the leather (i.e., make the leather impermeable to water without affecting its permeability to water vapor) but at the same time do not adversely affect feel, dyeability, grain pipeyness, and subsequent finishing.
It has now surprisingly been found that the use of ester urethanes having a certain composition in which no polyether groups are present provides the leather with a useful combination of desirable properties.